In the absence of any signal, the frequency band is considered to be free. In the converse case, it is considered to be occupied.
With the development of telecommunications, optimization of the use of the radio spectrum is becoming an increasingly significant concern for telecommunications operators.
Indeed, the number of applications using the radio spectrum is growing and numerous new applications are also becoming increasingly greedy for frequencies such as for example UMTS (for Universal Mobile Telecommunications System in English) and Wifi.
Moreover, a study conducted in November 2002 by the FCC (for Federal Communications Commission in English) has revealed the fact that the radio spectrum is not used effectively. Thus, at a given instant and for a given geographical place, there is a large probability of finding free frequency bands in the radio spectrum.
Access to the radio spectrum for the provision of services makes it necessary to benefit from a licence. However, the FCC has recently authorized free access, that is to say without a licence, to those of the free frequency bands whose use is normally intended for television. Access to these particular free frequency bands must be effected from radio systems able to implement the principles of the field of cognitive radio, that is to say be capable of collecting certain parameters on their environment so as in particular to detect free frequency bands.
Consequently, optimization of the use of the radio spectrum involves detecting free frequency bands.
Several solutions have already been proposed for detecting free frequency bands.
Some of these solutions are based on geo-location. The position of a terminal which incorporates a GPS function (for Global Positioning System in English) is given by satellite. The terminal accesses databases which, as a function of its position, provide it with the state of the radio spectrum. Certain terminals can be preprogrammed and can incorporate databases directly. The drawback of solutions of this type is that they are constraining and of limited use. Indeed, the terminal must be able to determine its geographical position and interrogate a database or else be used in a predetermined geographical zone corresponding to the incorporated database.
Other solutions such as radiometric (or energy-based) detection are based on detecting a signal in the noise on a radio frequency band. These techniques consist in measuring the energy of the signal. If the energy is greater than a certain predetermined threshold taking account of the noise level, then it is deduced therefrom that a telecommunications signal is present on the frequency band considered. These solutions exhibit the drawback of requiring a priori knowledge about the noise level.
Another type of solution for detecting a signal on a radio frequency band is based on revealing the cyclostationary nature of the telecommunications signals. Cyclostationary signals possess statistics (mean and autocorrelation) that are time dependent and periodic over time. The frequency of the autocorrelation function of a cyclostationary signal is cyclic. The noise is for its part modeled as a stationary signal whose statistics are independent of time. The distinction between a telecommunications signal and noise is made by applying a test of cyclic frequencies. If the cyclic nature of the frequency of the autocorrelation function of the signal received is revealed then it may be concluded that a telecommunications signal is present on the frequency band considered. The drawback of the existing solutions relying on the cyclostationary nature of telecommunications signals is of being able to detect only one cyclic frequency at a time. These solutions also exhibit the drawback of requiring a priori knowledge of the cyclic frequencies of the signals which may appear on the frequency band considered.
The drawback of the existing solutions is that they do not allow the detection on a given frequency band of a telecommunications signal without having a little a priori knowledge either about this signal or about the noise.
There therefore exists a requirement for a technique making it possible to detect a possible telecommunications signal on a given frequency band without a priori knowledge of this signal.